1. Field of the Invention
The present invention relates generally to audio processing, and more particularly to techniques for active noise cancellation.
2. Description of Related Art
An active noise cancellation (ANC) system in an earpiece based audio device can be used to reduce background noise. The ANC system forms a compensation signal adapted to cancel background noise at a listening position inside the earpiece. The compensation signal is provided to an audio transducer (e.g. a loudspeaker) which generates an “anti-noise” acoustic wave. The anti-noise acoustic wave is intended to attenuate or eliminate the background noise at the listening position via destructive interference, so that only the desired audio remains. Consequently, the combination of the anti-noise acoustic wave and the background noise at the listening position results in cancellation of both and hence a reduction in noise.
ANC systems may generally be divided into feedforward ANC systems and feedback ANC systems. In a typical feedforward ANC system, a reference microphone provides a reference signal based on the background noise captured at a reference position. The reference signal is then used by the ANC system to predict the background noise at the listening position so that is can be cancelled. Typically, this prediction utilizes a transfer function which models the acoustic path from the reference position to the listening position. Active noise cancellation is then performed to form a compensation signal adapted to cancel the noise, whereby the reference signal is filtered based on the transfer function.
The performance of the ANC system is constrained by the latency (or delay) introduced during the formation of the compensation signal. The latency limits the amount of noise attenuation achievable by the ANC system. For feedforward systems, excessive latency makes the anti-noise signal arrive too late to effectively cancel the noise signal, resulting in unsatisfactory cancelation at higher frequencies. For feedback systems, excessive latency can cause the closed-loop system to become unstable when the feedback gain is increased, effectively limiting the gain to a small value, which results in degraded noise attenuation performance. In either case, the resulting residual noise can interfere with the listening experience of desired sound, and is annoying. In some instances, the latency may result in the generation of an anti-noise acoustic wave that constructively interferes with the background noise at the listening position. In such a case, the combination of the anti-noise acoustic wave and the background noise may result in an increase in the noise at the listening position, rather than a decrease.
In order to achieve a relatively low latency, an ANC system may be implemented using analog filter circuitry. The analog circuitry filters and inverts the analog reference signal received from the reference microphone to form an analog compensation signal which is then provided to the loudspeaker. Although low latency can be achieved, the use of analog filter circuitry to perform active noise cancellation results in a number of drawbacks. For example, it can be difficult to achieve high precision or accuracy using analog filter components due to component variation. As a result, the component variation limits the overall noise cancellation performance of the ANC system. In addition, analog filter components are susceptible to drift and aging which can cause the performance to worsen over time. Finally, it can be difficult to change component values to adapt to various situations or to provide the user more flexibility in the amount or the nature of the noise attenuation, which makes analog circuitry less flexible in practice than digital solutions.
It is therefore desirable to provide low latency active noise cancellation techniques which can also address the problems associated with analog filter circuitry.